Tell something about reactive model

- Functional Reactive Programming (FRP) integrates time flow and compositional events into functional programming. This provides an elegant way to express computation in domains such as interactive animations, robotics, computer vision, user interfaces, and simulation.

- It is about "data-types that represent a value 'over time' ". Conventional imperative programming captures these dynamic values only indirectly, through state and mutations. The complete history (past, present, future) has no first class representation. Moreover, only discretely evolving values can be (indirectly) captured, since the imperative paradigm is temporally discrete. In contrast, FRP captures these evolving values directly and has no difficulty with continuously evolving values.

- FRP: model "dynamic -continuous- values" (values changing overtime) and how to compose them:

>> Behaviour (also called flow) : the value function over time (t --> v(t)).
>> Events (switchers) :  perform a change on Behavior discretely.